Many implementations of 5G base stations are expected to utilize so-called analog beamforming. This is due to the higher complexity, mostly from a hardware perspective, of implementing so-called digital beamforming. The latter imposes fewer functionality restrictions but is rather more costly to realize.
As used herein, “beamforming” means that a transmitter can amplify transmitted signal power in selected directions, while suppressing transmitted signal power in other directions. Correspondingly, a receiver can amplify signals received from selected directions while weakening unwanted signals in other directions. Analog beamforming in this context means that beamforming can only be applied to one direction or a limited set of directions at a time (e.g. in one OFDM symbol) by each transmitter/receiver. An array of multiple transmit antennas or receive antennas must be used to transmit or receive in multiple directions at the same time. To beamform, a signal is transmitted from multiple transmit antennas, but with individually adjusted phase shifts or time delays, which effectively creates a beam in the resulting transmit radiation pattern of the signal—e.g., through controlled constructive and destructive interference of the phase-shifted signals from individual antenna elements. The beam direction depends on the phase shifts of the antenna elements. Similarly, in the case of a receiver, phase shifts between antenna elements can be used to steer the maximal antenna sensitivity toward a desired direction.
Beamforming allows the received signal to be stronger for an individual connection, thereby enhancing throughput and coverage for that connection. It also enables a reduction in the interference from unwanted signals, thereby enabling several simultaneous transmissions over multiple individual connections using the same time-frequency resource, so called spatial multiplexing or MIMO using either a single user, SU-MIMO, or multiple users, MU-MIMO.
An important problem with beamforming is to decide which beam(s) (i.e., which direction(s)) to use for transmission and/or reception. To support base station beamforming, a number of reference signals may be transmitted in different beam directions, respectively, from the base station. Each User Equipment (UE) can measure these reference signals and report the measurement results to the base station. The base station can then use these measurements to decide which beam(s) to use for data transmission to one or more UEs. As further described herein, a network can use a combination of persistent and dynamic reference signals for this purpose.
The persistent reference signals, denoted herein as beam reference signals (BRS), are transmitted repeatedly in a large number of different beam directions. This allows a UE to measure the BRS when transmitted in different beams, without any special arrangement or instruction for that UE from perspective of the base station. The UE reports the received powers for different BRS back to the base station, along with the index of the BRS, given for example by the BRS sequence and the time and frequency position of the particular BRS. By reporting a BRS index and an associated received power of that BRS, the UE is effectively reporting its preferred beam. The UE may report a list of BRS indices and associated powers, for example, its top eight strongest BRSs.
The base station can then transmit dedicated reference signals to a particular UE, using one or more beams or beam directions that were reported as strong for that UE. These are dedicated reference signals and may thus only be present when the UE has data to receive, and they give more detailed feedback information of the beamformed channel, such as co-phasing information of the polarizations and the recommended transport block size, as well as the transmission rank in case of spatial multiplexing. Since the BRS is transmitted repeatedly over a large number of beams, the repetition period should be relatively long, to avoid using too much resource overhead for the BRS transmissions.
The dynamic reference signals, denoted herein as channel-state information reference signals (CSI-RS) or beam-refinement reference signals (BRRS), are transmitted only when needed for a particular connection. The CSI-RS is the 3GPP terminology for a schedulable, and typically UE-specific, reference signal that can be utilized for various purposes such as channel acquisition and beam management. Herein the terminology of BRRS is used when referring to such a reference signal when used for receive beam selection and tracking. CSI-RS is used when referring to such a reference signal when used to acquire that channel state information for feedback of e.g. preferred modulation and coding scheme (MCS). The decisions of when and how to transmit the CSI-RS are made by the base station and signaled to the involved UEs using a measurement grant or configuration message. When the UE receives a measurement grant it measures on the corresponding CSI-RS. The base station can choose to transmit CSI-RS to a UE using only beam(s) that are known to be strong for that UE, to allow the UE to report more detailed information about those beams. Alternatively, the base station can choose to transmit CSI-RS also using beam(s) that are not known to be strong for that UE, for instance to enable fast detection of new beam(s) in case the UE is moving.
The 5G base stations transmit other reference signals as well. For example, they transmit a demodulation reference signal (DMRS) when transmitting control information or data to a UE. Such transmissions are typically made using beam(s) that are known to be strong for that UE.
In 4G systems, discovery reference signals (DRS) may be used for the same purpose as BRS, as described above. Hence, the LTE UE is configured to perform received power measurement on a set of different DRS signals and report the associated DRS index and measured power for the eight DRS measurements with highest power.
Beamforming is not restricted to base stations. It can also be implemented in the receiver of the UE, further enhancing the received signal and suppressing interfering signals. The UE may also implement transmit beamforming. Similar to a base station, analog beamforming can be used in the UE, which means that the UE can only receive/transmit from/to one direction at a time, unless multiple receivers/transmitters are available. When operating with the 5G base stations, a UE with analog receive beamforming can measure the BRS using different UE receive beams, and then choose the UE receive beam(s) that provides the highest BRSRP (Beam Reference Signal Received Power).
Known implementations of analog beamforming in wireless communication networks do not provide mechanisms for robust management of network and UE beams.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.